How well does this extra-firm mattress isolate motion for couples?

It does an honest job, scoring 8.6, but be aware of the build. This is a connected Bonnell innerspring, not individually wrapped coils, so the wires share load and carry more movement across the surface than a foam or pocket-coil bed. The same springy liveliness that makes it easy to move on is what a partner can feel. Most couples are fine; a very light sleeper beside a restless partner will notice the bounce more.